The objective of the proposed research is the development of an understanding of the role of maternal and embryonic teratogen metabolism and its application to the development of a simple, reliable teratologic screening test. This research combines whole embryo culture with current pharmacological methods. Species, strains and individuals are known to possess different genetically programmed biotransformational pathways. We hypothesize that their individual teratogenic responses to drugs and chemicals are shaped by these quantitative and qualitative metabolic differences. Embryos are known to have negligible biotransformational abilities. We might therefore expect a rat embryo to develop defects characteristic of another species if presented with the teratogenic metabolites generated by that other species. In our test system, early somite rat embryos are cultured in media containing a teratogen, hepatic enzyme systems (S-9) and the cofactors required for a specific metabolic process. S-9 can be obtained from livers of rats, mice, rabbits, monkeys or human cadavers. Cultures are incubated for 24 to 48 hours during which time control growth and development approximate normal limits. Analysis includes morphology, histology and protein and DNA content and synthesis. Using this system, we have demonstrated that cyclophosphamide requires biotransformation in order to exert teratogenic effects, as with its alkylating and cytotoxic properties. We plan to investigate other known teratogens with differential species effects including DON, DMBA, thalidomide, rifamapicin, procarbazine and cytochalasin D to determine whether they require enzymatic bioactivation for teratogenic activity and whether the pattern of malformations they induce is determined by the genetic background of the S-9 donor source. By substitution of various cofactors and inhibitors, we hope to define crucial activational/inactivational processes in teratogenesis. If successful, this test system would be free of much of the "relevancy" objection to current animal testing schemes.